1. Field of the Invention
The present invention relates to an electro-acoustic micro-converter for generating sound from an electronic input signal, in particular for a device worn in an ear of a user, comprising a device housing with a sound output in which a membrane assembly is suspended with freedom of movement, the membrane assembly comprising an acoustic membrane and a coil element connected thereto, wherein the coil element is provided with connecting means for receiving the electronic input signal and extends in a coil gap, and wherein the device housing comprises magnet means which maintain a magnetic field in the coil gap, the magnet means comprising a permanently magnetic magnet body.
Such a converter is also referred to as a transceiver or receiver and is normally applied for the purpose of acoustic reproduction of sound on the basis of an electronic audio signal. As such, electro-acoustic converters find their way into electronic devices of diverse nature, such as for instance (mobile) phones, computers and, by no means least, hearing aids and other audio devices worn in the ear and having a device housing which is worn wholly or largely concealed in the ear, such as earphones and active audio filters.
A great challenge in the case of this latter group is the far-reaching miniaturization of the components received therein because of the limited placing space in an auditory canal of a user. This miniaturization also applies for the electro-acoustic converter applied therein, which in this case is therefore normally referred to as miniature or micro-converter. An extreme degree of miniaturization of an electro-acoustic converter does however quickly result in a limitation of the surface area available for the active membrane, and thereby in a reduction in the maximum acoustic output power that can be produced. Miniaturization moreover involves the danger of an audible deterioration in the sound quality.
Micro-converters of the so-called balanced armature type are usually applied for hearing aids. Such a micro-converter is based on magnetic forces exerted between a pair of magnetic poles of permanent magnet bodies located opposite each other on a magnetically susceptible armature element. The armature element is suspended here for movement between a pair of coil elements to which an electronic audio signal is supplied. A rapidly changing electronic audio input signal is sent through the coils, whereby the armature element suspended therebetween is set into vibration and transmits its movement to an acoustic membrane connected to the armature element. The membrane thus set into vibration in turn produces sound which is representative of the supplied audio signal and which can exit from the sound output.
A relatively high efficiency can be achieved with such a device by selecting an eigenfrequency of the armature assembly in an efficient acoustic frequency range. This relatively high efficiency does not however apply at all outside this range, whereby such converter is intended mainly for a high acoustic output power in a relatively narrow frequency range. For more wideband applications, such as for pure audio reproduction, i.e. over a relatively wide frequency spectrum, such a balanced armature converter is therefore a less likely solution. A balanced armature converter is moreover relatively expensive due to the relatively large number of components applied therein and the manufacturing steps required therefore. Particularly the exact placing and alignment of the armature element precisely in the centre of the air gap between the two magnetic poles, in addition to the magnetic balance required between these two latter, require high precision and result in an increase in the cost price.
A more natural and therefore clearer audio reproduction can be achieved with a converter of the type stated in the preamble, which is also referred to as the so-called moving coil type. Converters of this type are therefore not infrequently applied in loudspeakers of hi-fi systems, in headphones and earphones and in mobile electronic products such as telephones and computers. A characteristic of this type of converter is that the acoustic membrane is coupled to an electronic coil element which is suspended with freedom of movement in a permanent magnetic field maintained by a permanent magnet body. This type of converter is generally less well suited for miniature applications, such as in a device worn in the ear, because of the generally inadequate conversion efficiency. The converter therefore requires relatively large dimensions which are not consistent with a miniature application, such as in the ear, and moreover consumes a relatively large amount of power, which is likewise disadvantageous for a miniature application such as in the ear, which normally is wholly dependent for its electrical power supply on the relatively modest capacity of a battery or accumulator.
2. Description of the Related Art
An example of a miniature electroacoustic moving coil converter is described in the American patent application USP 2008/0044044. This known micro-converter comprises a device housing having therein a permanent magnetic body provided with pole bodies which together maintain a permanent magnetic field through a coil element extending from an acoustic membrane suspended in freely movable manner. For the purpose of increasing the acoustic output power, this known converter here comprises not one but two acoustic membranes, each with such a coil element in the magnetic field of the magnet means. A drawback hereof is however an increased complexity of the device as a whole and the necessity of a sound channel in which the sound waves of both membranes are brought together acoustically and carried to a shared sound output. The device hereby becomes more voluminous. For application deep in an auditory canal of a user, such as in the case of a device worn in the ear and in particular a completely-in-canal (CIC) hearing aid, the known construction is therefore far from optimal.